For a couple of years I used to do fasts because of religious reasons ... 3-days every couple of weeks, 7-days every 2-3 months ... did 34-days once (was trying for 40-days but collapsed at work - it's lucky I worked in a hospital) - and these were fasts with water only.
I wound up having to take thyroid for about 10 years, and having problems with episodes of hypoglycemia for over 30 years (still get episodes from time to time)! Problems I never had before.
If you do decide to do a fast - do it under competent medical supervision - and, as has been mentioned ... how you break the fast is as important as anything else you do during the fast.
Unless you have a proven (as in a doctor says you need to) medical reason to need to screw up your body's metabolism through fasting - I wouldn't.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain